SEA-FISHERIES LABORATORY. 327 



diatoms. Putter, however, states that the amount found 

 in the alimentary canal is far too small, and that a 

 Calanus (sp. not given) would require 9,750,000 Thalas- 

 siosira nana per day, if it depended on the plankton. The 

 alimentary canal is, however, often found full of algal 

 cells and debris, and we do not know how many times it 

 can be filled and emptied in 24 hours. Allowing for this, 

 however, it may still be impossible for a Copepod to obtain 

 sufficient food in solid form if Putter's estimations of the 

 total amount required are correct. There are many other 

 examples cited by Putter representing the following 

 groups : Protozoa, Porifera, Echinodermata, Coelen- 

 terata, Mollusca, Crustacea, Tunicata and Fishes ; and if 

 the figures given are correct, it is evident that food must 

 be absorbed from the solution in sea-water. The balance 

 of biological evidence is in favour of solid food being 

 necessary, and hence it is only a question of an additional 

 method of obtaining food, and not of a substitution of 

 liquid for solid food. The amount of organic carbon 

 dissolved in the sea-water was stated by Putter in '07 to 

 be 65 mg. per litre. Henze and Raben have brought this 

 figure down to about 3-6 mg. per litre only. Allowing, 

 however, for this great change, there still appears to be 

 more carbon present in solution (in organic compounds) 

 than in the plankton. It will be seen that the amount of 

 organic carbon was considerably over-estimated, and it 

 remains therefore for the figures showing daily food 

 requirements to be carefully checked. 



The plankton still remains as the primary food 

 supply, for even if it is shown that the planktonic 

 organisms are insufficient as food, the organic carbon 

 compounds present in solution can be traced, as the 

 products of metabolism, to the phyto-plankton which is 

 their ultimate source. 



